Old corrugated containers (OCC) make up a high percentage of the recycled paper used in the United States, a majority of which is made into new container board. With such a high reuse level, old corrugated containers are an extremely important raw material for the container board industry. A significant unused source of OCC are wax treated containers. These are corrugated containers which are coated or impregnated with wax and used to ship fruit, produce and other agricultural products, as well as nonagricultural products such as bulk nail, screw, and bearing boxes. With waxed corrugated representing about 1.3 million tons or about 5% of the shipped corrugated in 1995, it represents an attractive fiber source. What makes this source even more attractive is the fact that waxed corrugated is typically produced from virgin fiber and its use is concentrated in a handful of industries; thus it is a high quality fiber source that would be relatively inexpensive to collect. Currently, waxed corrugated is not accepted for recycling. It is considered too contaminated and is sorted out for landfilling or sold for its b.t.u. value. The problem is that waxed corrugated causes a serious contamination issue for a mill. The released wax tends to collect on equipment and in the finished paper products causing problems with mill operations and affecting product quality. To reasonably increase the level of waxed OCC used by the paper industry to the point where mills will accept and possibly seek out waxed containers, great strides in new technology for controlling wax in the recycle plant will have to be introduced.
Wax is a generic term widely applied to a diverse group of natural and synthetic products which have similar physical characteristics; they are plastic solids at ambient temperatures, and low viscosity fluids at moderately elevated temperatures. The chemical composition of waxes can be complex, containing a variety of molecular weight species and functional groups, or relatively simple, as in the case of some petroleum and synthetic waxes which are composed solely of hydrocarbons. Natural waxes are derived from various sources such as insects, animals, vegetables, minerals, and petroleum. Examples of synthetic waxes include low molecular weight polyethylene (mol. wt.&lt;600), solid polyethylene glycols, amide waxes, and Fisher-Tropsch waxes (low molecular weight polymethylene).
Depending on the specific application, waxed corrugated containers will be coated with blends of paraffin and micro waxes as well as blends that include synthetic waxes, polymers, and resins. Synthetic waxes such as polyethylene and Fischer-Tropsch are similar to paraffin wax in that they are composed of unbranched alkanes, but because they're polymeric materials they tend to have much higher molecular weights. For example, polyethylene waxes are composed of low molecular weight polyethylenes, and Fisher-Tropsch are low molecular weight polymethylenes. Melting points for these materials tend to be significantly greater than those for petroleum waxes (&gt;200.degree. F. for polyethylene and up to 220.degree. F. for Fisher-Tropsch polymers), thus their addition to a wax will produce higher melting points and often increased coating hardness. The most common polymer additive to be incorporated into wax coatings is ethylene-vinyl acetate (EVA). EVA is a random copolymer of ethylene (50-95%) and vinylacetate (5-50%). EVAs with softening points of up to 400.degree. F. are often used.
Wax-blend coatings may also include tackifying resins which are incorporated to increase the affinity of the wax coating for a substrate. In addition to the adhesional effects, the added resins can also lower a coating's softening and melting points. Tackifying resins include hydrocarbons, rosins and rosin derivatives, and polyterpenes. Hydrocarbon resins can be aliphatic or aromatic hydrocarbons. Aliphatic rosins include low molecular weight polymers and alkenes with 5-carbon backbones. Aromatic hydrocarbon resins are 9-carbon derivatives of polystyrene. Rosins are free acids and acids containing double bonds. They're naturally occurring resins which are found in pine trees. Derivatives are produced by polymerizing, hydrogenating, or estifying rosin. Polyterpenes are derived from alpha- or beta-pinene.
In the recycle plant, mill operators have defined three categories for the wax that is removed from the OCC at the repulper: free wax which is removed from boxes as large discrete particles, suspended wax which exist as small dispersed particles (in the 40 micrometer range), and adsorbed wax which coats the fibers. The distribution amongst these categories will depend on the repulping temperatures, the melting point of the coatings, and the temperature of the pulp slurry as it moves through the recycle plant. A typical recycle mill will repulp OCC at temperatures of around 115-140.degree. F. For some coated containers, this may be hot enough to melt or suspend the wax and completely defiber the waxed corrugated containers. The low viscosity of the suspended wax allows for the formation of small, discrete particles which move freely with the pulp slurry. These particles are removed at high efficiencies during the washing or dewatering of the pulp, but as the temperature drops, they can again solidify and adhere to fibers (adsorbed wax) and mill equipment. At the typical wax-based (contamination) level in OCC, this only periodically affects operations. However, when levels of treated containers increase (e.g., when grocery store bails which typically contain a high wax-coated container concentration enter the system), the higher levels of wax results in more frequent shutdowns (breaks and clean-ups) and lower quality product.
In addition to appearance problems from wax spotting in final products, the wax associated with the fiber can interfere with the performance of additives such as sizing agents and retention aids. It can also affect the handling, conversion, and performance of the final product. The wax lowers the paper-to-paper friction coefficient. In the industry, this is typically quantified as the tangent of the angle at which two paper surfaces will begin to slide free from each other, known as the angle of slip (AOS), or slide angle. There is strong evidence that wax is the primary cause of reduced AOS in board utilizing OCC. This reduced friction coefficient can create problems during conversion. For example, the slippery surfaces of the linerboard can cause the corners of the linerboard to get out of alignment and possibly tumble over, thus reducing the stack height of linerboard and slowing the converting operations. Wax adsorbed interferes with fiber-fiber bonding, which reduces product strength characteristics. Moreover, there is strong evidence that Scott bond and compression strength are reduced considerably by the presence of wax.
There are three major types of wax treated boxes found in bundles of OCC: impregnated, saturated, and curtain-coated. They all can produce contamination problems because all three types can contain wax that will soften or melt at repulping temperatures, hindering removal and promoting deposition on equipment. Approaches for dealing with wax contamination have been threefold. The problem may be addressed upon repulping with improved mechanical processes to aid separation, upon application of the initial coating of the paper to eventually be repulped by coating additives; or upon improved repulping with repulping additives. This invention focuses on improvements of the latter type.
Mechanical improvements to the recycling process to increase efficiency (the first type of solution to the problem) include the use of ultrasound in U.S. Pat. No. 4,045,243 and a high pressure stream of steam in U.S. Pat. No. 4,312,701.
Development of more readily repulpable coatings (the second type of solution to the problem) that can be easily separated from fiber and removed with conventional cleaning systems (i.e., screens and cleaners) is ongoing. For example, dispersant coating additives are generally described in WO 91/05107. Many polymeric additives to coatings have also been identified. Copolymers of acrylamide/acrylic esters have been disclosed in U.S. Pat. No. 3,262,838; the copolymerization product of styrene and the half ester formed by the half esterification of 1 mole of an alpha-beta unsaturated dicarboxylic acid has been disclosed in U.S. Pat. No. 3,287,149; butadiene-methyl methacrylate copolymer latex is disclosed in U.S. Pat. No. 4,117,199; a wax composition is disclosed in U.S. Pat. No. 5,539,035 and a variety of other polymeric coating additives are disclosed in U.S. Pat. Nos. 5,491,190; 5,658,971 and 5,654,039.
To aid the repulping process, efforts have also been focused on the development of more readily repulpable hot melt adhesives, as disclosed in U.S. Pat. No. 5,541,246 for example.
Examples of the third type of solution, the development of repulping process additives include EP 0,568,229 A1 which discloses a hydrophobically modified associative polymer including hydrophobically substituted polyethylene oxide polymers; U.S. Pat. No. 4,643,800 which discloses use of a substituted oxyethylene glycol non-ionic surfactant and a water-soluble, low molecular weight polyelectrolyte dispersant; alkylamine polyethers for foam control disclosed in U.S. Pat. No. 4,483,741 and U.S. Pat. No.1,628,931 which discloses use of trisodium phosphate. Moreover, additives to the repulping process for the removal of ink include long-chain alcohols as disclosed in U.S. Pat. No. 5,500,082 and surface-active poly(ethers) in U.S. Pat. No. 4,518,459.
Dispersion of waxes is a problem in many industrial processes, aside from the repulping process in the pulp and paper industry. Dissipation of waxy dispersions with cationic, anionic or non-ionic emulsifying agents have been disclosed in U.S. Pat. No. 3,537,990. Wax coagulation has been effected by cyanamide derivatives as disclosed in U.S. Pat. No. 4,629,477; by water-soluble non-ionic emulsifiers in U.S. Pat. No. 3,822,178; by a plant glycoside dispersion stabilizer in U.S. Pat. No. 5,403,392; by means of molybdenum-containing coagulants in U.S. Pat. No. 5,324,437 and by a mechanical technique in U.S. Pat. No. 3,940,334.